Processes and devices for sterilizing contaminated objects

ABSTRACT

The present invention relates to a process for sterilizing a contaminated object. The process comprises the insertion of the object in a sterilization chamber having at least one discharge tube in communication therewith. Then, the tube is fed with a liquid or gas stream, and the stream is subjected to an electric field so as to generate a plasma, thereby exposing the contaminated object to the action of sterilizing species that are present in a post-discharge zone or in a zone of excitation of the plasma. A device for carrying such a process is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of PCT InternationalPatent Application No. PCT/CA2003/001116, filed on Jul. 24, 2003, whichclaims priority on Canadian Patent Application No. 2,395,659, filed onJul. 26, 2002. Each of the above-mentioned applications are incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The present application relates to devices and processes allowingsterilization of contaminated objects. In particular, the presentinvention relates to high performance devices and processes forsterilizing contaminated objects, such as medical instruments andaccessories, by utilizing a plasma gas, also called ionized gas. Theseprocesses and devices can carry out sterilization of an importantsurface of contaminated objects, in a single treatment.

BACKGROUND OF THE INVENTION

By reason of standards that are more and more rigid for thesterilization of objects, such as in the medical field and the foodindustry, there is an increasing need for new high performance devicesand processes that are free of the limitations and/or disadvantages ofknown devices.

The devices that use plasma sterilizing processes, in spite of theirexcellent performances, have not yet achieve a significant breakthroughin the sterilization market that traditionally involves the utilizationof devices that use vapor and/or chemical treatments.

U.S. Pat. No. 6,707,254 describes a sterilization process that uses aplasma post-discharge, this discharge being carried out in a gasmixture, in which the main gas is nitrogen, or argon. Such a mixture isdescribed as advantageously containing between 0.5% and 20% O₂, in orderthat sterilization time be as short as possible.

The effect of UV on DNA has been studied in an article by Moisan et al.,which appeared in International Journal of Pharmaceutics, vol. 226 pp1-21 (2001).

These prior art processes have limitations, for example with respect tothe homogeneity of distribution of sterilizing species and, consequentlywith respect to contaminated objects, and with respect to the totalsurface of the contaminated objects that can be treated in the samechamber.

Moreover, several of the known processes and devices tend to damage ordeteriorate the objects to be sterilized. After being submitted to aplurality of such treatments these objects can be considerably damaged.

There was therefore a need for new processes and devices that are freeof at least one limitation of the devices of the prior art.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to overcome theabove-mentioned drawbacks and to provide new processes and devices forsterilizing contaminated objects.

According to a first aspect of the invention, there is provided aprocess for sterilizing a contaminated object in a sterilization chamberthat is provided with at least one discharge tube. The discharge tube(s)reach(es) the sterilization chamber and is (are) fed with a liquid orgas stream feed. The contaminated object is subjected in thesterilization chamber to the action of sterilizing species that arepresent in a post-discharge zone or in a zone of plasma excitation thatis generated, at the level of the discharge tube(s), by subjecting thestream to an electric field. The stream is adjusted by controlling theflow rate and/or the gas pressure in the chamber, so as to maximizeUltra Violet (UV) radiation intensity.

According to a second aspect of the invention, there is provided aprocess for sterilizing a contaminated object comprising:

-   -   inserting the object in a sterilization chamber having at least        one discharge tube in communication therewith,    -   feeding the tube with a liquid or gas stream, and submitting the        stream to an electric field so as to generate a plasma, thereby        exposing the contaminated object to the action of sterilizing        species that are present in a post-discharge zone or in a zone        of excitation of the plasma,    -   wherein the stream is adjusted by controlling its flow rate        and/or gas pressure in the chamber, so as to maximize Ultra        Violet (UV) radiation intensity.

It has been found that a process, as defined in the first or secondaspect of the invention, for sterilizing a contaminated object bymaximizing the UV radiation intensity through the control of the flowrate and/or gas pressure in the chamber, permit to obtain improvedresults with respect to the sterilization. In fact, such processes havedemonstrated a high performance sterilization. It has been demonstrated,for a particular gas, that by obtaining a preferred range of values ofpressure (that permit to maximize the UV radiation intensity) bymeasuring (recording) the UV emission intensity as a function of thepressure, and/or by obtaining a preferred range of values of flow rate(that permit to maximize the UV radiation intensity) by measuring the UVemission intensity as a function of the flow rate, and by applying atleast one range (and preferably both ranges) of these values (ranges ofpressure and flow rate) during a sterilization process, permit tooptimize the sterilization process. In fact, by applying these preferredranges of pressure and of flow rate, which each correspond to maximizedvalues of UV radiation intensity, the time required for obtainingsterility of the contaminated object can be considerably reduced.

According to a third aspect of the invention, there is provided aprocess for sterilizing a contaminated object in a sterilization chamberprovided with at least one discharge tube. The discharge tube(s) is incommunication with the sterilization chamber and is (are) supplied witha liquid or gas stream. The contaminated object is subjected in thesterilization chamber to the action of sterilizing species that arepresent in a post-discharge zone or in a zone of plasma excitation thatis generated at the level of the discharge tube(s) by treating thestream in an electric field. The ratio R=(CDT)/(CSC), wherein CDTrepresents the cross-section of the discharge tube in communication withthe sterilization chamber or the sum of the cross-sections of thedischarge tube(s). CSC represents the cross-section of the sterilizationchamber (CSC), confirms the relation 0.05<R<0.70. The stream is adjustedby controlling the flow rate and/or gas pressure in the chamber, so asto maximize Ultra Violet (UV) radiation intensity.

It has been found that the process as defined in the third aspect of theinvention was very effective for sterilizing a contaminated object. Infact, by maximizing the UV radiation intensity through control of theflow rate and/or gas pressure in the chamber, and by obtaining a ratio(CDT)/(CSC) comprised between 0.05 and 0.70, improved results withrespect to the sterilization process were noted.

According to a fourth aspect of the invention, there is provided aprocess for sterilizing a contaminated object comprising:

-   -   inserting the object in a sterilization chamber having at least        one discharge tube in communication therewith,    -   feeding the tube with a liquid or gas stream, and subjecting the        stream to an electric field so as to generate a plasma, thereby        exposing the contaminated object to the action of sterilizing        species that are present in a post-discharge zone or in a zone        of excitation of the plasma,    -   wherein the stream consists of helium, neon, argon, krypton,        xenon, or mixtures thereof.

It has been found that by using a process as defined in the fourthaspect of the invention a high performance sterilization is obtained anddeterioration or erosion of the treated object is considerably reduced.In fact, by using a gas stream consisting of helium, neon, argon,krypton, xenon, or mixtures thereof, the presence of oxidizing speciesbeing avoided, thereby considerably reducing damages that can be broughtto the treated object. The person skilled in the art would clearlyrecognize that the expression “consists of” (or “consisting of”) whenreferring, as example, to a particular gas means that this particulargas is the only one used in the process. In fact, in such a case theparticular gas is used as provided from a supplier. The person skilledin the art would also understand that there may be some minor impuritiescontained in the gas provided from the supplier. However, it ispreferable to use high purity grade gases in such process. It is alsopossible to use or prepare a mixture of more than one particular gasprovided by a supplier. The term “pure”, as used herein when referringto a particular gas, is used as a synonym of the expression “consistsof” (“or consisting of”).

According to another aspect of the invention, there is provided asterilization device allowing the implementation of one of the processesas previously defined, comprising a source of plasma associated with oneof the walls of the sterilization chamber by means of at least onedischarge tube in which there is injected a gas or a mixture of gaseseventually producing the plasma. The chamber comprises the object to besterilized, and a vacuum pump carries the gases in the chamber andmaintains therein a reduced pressure. The source of plasma alsocomprises an electric field applicator. The ratio R=(CDT)/(CSC), inwhich (CDT) represents the cross-section of the discharge tube or thesum of the cross-sections of the discharge tube(s) in communication withthe sterilization chamber and (CSC) represents the cross-section of thesterilization chamber (CSC), confirms the relation 0.05<R<07.

It has been found that such a device permits to efficiently carry outany of the processes previously defined.

In the processes of the invention, the plasma generating electric fieldis preferably a high frequency field. For example, the frequency can befrom about 10 Megahertz to about 3 Gigahertz. The frequency can bepreferably of about 100 and to about 2450 MHz, and more preferably at433, 915 or 2450 MHz which are frequencies authorized for industrial,medical and scientific (ISM) purposes),

The gas stream flow rate can be adjusted to a value that is between 10and 5000 standard cm³ per minute (sccm), and preferably to a valuebetween 50 and 3000 sccm. Advantageously, the pressure that is generatedinside the sterilization chamber is between 0.05 and 10 torrs or 0.1 and10 torrs. Alternatively, a pressure between 0.1 and 4 torrs or between 1and 8 torrs can be used for certain gases. The steps of such processescan involve a pulsed gas in an electric field that is appliedcontinuously, a pulsed electric field in a continuous gas stream, apulsed gas in a synchronously pulsed electric field, a gas change or acombination thereof. The sterilizing species can comprise photons,radicals, atoms, molecules or combination thereof. Preferably, thesterilizing species comprise a major portion of photons and/or radicals.The object to treated can be contaminated with micro-organisms such asviruses, spores, bacteria, fungi, molds, prions or combination thereof.

The processes of the present invention can be carried out at atemperature inside the sterilization chamber of 60° C. or less andpreferably 50° C. or less. More preferably, this temperature is about30° C. These processes can lasts between 10 minutes and 4 hours. Theycan be carried out in isolated or repeated manner in a multi-stepsequential procedure.

In the processes according to the first, second and third aspects of theinvention, the gas stream can comprise at least one component selectedfrom the group consisting of molecular oxygen, nitrogen, neon, argon,krypton, xenon, helium, oxygen, carbon monoxide, carbon dioxide, N₂O,gases of formula NO_(x) wherein x represents 1, 2 or 3, air, andmixtures thereof. According to a preferred embodiment, the gas streamcomprises molecular oxygen, preferably from 0.1 to 10% and morepreferably from 0.2 to 5%. Alternatively, the gas stream can comprise atleast 0.04% of molecular oxygen. According to another preferredembodiment of the invention, the gas stream comprises molecular oxygenand a component selected from the group consisting of nitrogen, neon,argon, krypton, xenon, helium, oxygen, carbon monoxide, carbon dioxide,N₂O, gases of formula NO_(x) wherein x represents 1, 2 or 3, air, andmixtures thereof. According to another preferred embodiment of theinvention, the gas stream comprises nitrogen, argon and helium, inaddition to molecular oxygen. Alternatively, it can comprise nitrogen,argon and nitrogen dioxide, in addition to molecular oxygen. The controlof flow rate and/or gas pressure in the chamber, so as to obtain a UltraViolet (UV) radiation of maximum intensity, can be carried out byselecting preferred ranges of values for at least one of these twoparameters (flow rate and gas pressure). As an example, it can be doneby selecting at least 2 or 3 (preferably at least 4) predetermined orconstant values of flow rate and then measuring the UV emissionintensity for each of these predetermined or constant values of flowrate as a function of pressure so as to select the parameters thatpermit to maximize the UV radiation intensity. Alternatively, it canalso be done by selecting at least 2 or 3 (preferably at least 4)predetermined or constant values of pressure and then measuring the UVemission intensity for each of these predetermined or constant values ofpressure as a function of the flow rate so as to select the parametersthat permit to maximize the UV radiation intensity.

In preferred embodiments of the processes according to the first, secondand third aspects, the gas stream can have one of the followingcompositions:

-   -   from 0.04 to 30% O₂; from 0.05 to 99.91% nitrogen; and from 0.05        to 99.91% argon;    -   from 0.04 to 30% O₂; from 0.05 to 99.91% nitrogen; and from 0.05        to 99.91% krypton;    -   from 0.04 to 99.90% O₂; from 0.05 to 99.91% nitrogen; and    -   from 0.05 to 99.91% xenon or from 0.05 to 99.91% neon; or from        0.04 to 98.5% O₂; from 0.05 to 99.6% nitrogen; and from 0.05 to        99.6% xenon or from 0.05 to 99.6% neon.

Alternatively, the gas stream may consist of NO₂, nitrogen, or a mixtureof oxygen and nitrogen and the pressure that is generated inside thesterilization chamber is then advantageously between 2 and 8 torrs.

In the processes of the first, second and third aspects of the inventionthe gas stream can alternatively consists of helium, neon, argon,krypton, xenon, or mixtures thereof. Preferably, the gas stream consistsof argon.

In the processes of the first, second and fourth aspects of theinvention, the ratio ratio R=(CDT)/(CSC) can be comprised between 0.01and 0.70. CDT represents the cross-section of the discharge tube or thesum of the cross-sections of the discharge tubes and CSC represents thecross-section of the sterilization chamber. Preferably, 0.09≦R≦0.60,more preferably 0.15≦R≦0.5, even more preferably 0.2≦R≦0.40.Alternatively, R can have a value between 0.05 and 0.20.

In the process according to the third aspect of the invention, whereinthe sterilization chamber can be perpendicular to the direction of thegas stream feeding the discharge tube and cross-section (CSC)representing the cross-section of the chamber in communication with thedischarge tube and which is perpendicular to the plasma current.Preferably, 0.09≦R≦0.60, more preferably 0.15≦R≦0.5, even morepreferably 0.2≦R≦0.40. In the sterilization device of the presentinvention, the electric field applicator can be of the surfatron orsurfaguide type. The sterilization chamber can be entirely or partiallymade of borosilicate or aluminum. The sterilization chamber preferablycomprises a support for the objects to be sterilized. The sterilizationchamber is preferably cylindrical or parallelepipedal, andadvantageously this sterilization chamber has a substantially constantcross-section.

According to another aspect of the present invention there is provided aprocess for sterilizing a contaminated object comprising exposing theobject in a sterilization chamber to a plasma that is produced in atleast one discharge tube that reaches the chamber, from a gas streamcontaining at least one gas of the group consisting of oxygen and raregases such as helium, neon, argon, krypton and xenon. The sterilizationchamber contains at least one discharge tube that reaches the chamber.

BRIEF DESCRIPTION OF DRAWINGS

Further features and advantages of the invention will become morereadily apparent from the following description of preferred embodimentsas illustrated by way of examples in the appended drawings wherein:

FIG. 1 illustrates the relative intensity variation of UV photonemission as a function of the percentage of O₂ in a N₂/O₂ mixture, in aprocess for sterilizing a contaminated object according to a preferredembodiment of the invention;

FIG. 2 illustrates the relative intensity variation of UV photonemission as a function of the percentage of O₂ in a N₂/O₂ mixture, in aprocess for sterilizing a contaminated object according to anotherpreferred embodiment of the invention;

FIG. 3 illustrates the number of spore B. subtilis survivors as afunction of time in a process for sterilizing a contaminated objectaccording to another preferred embodiment of the invention, wherein apost-discharge of a N₂/O₂ mixture is used to sterilize the contaminatedobject;

FIG. 4 illustrates the number of spore B. subtilis survivors as afunction of time in a process for sterilizing a contaminated objectaccording to another preferred embodiment of the invention, wherein apost-discharge of argon is used to sterilize the contaminated object;

FIG. 5 illustrates a sterilization device as described in U.S. Pat. No.6,707,254, which is hereby incorporated by reference;

FIG. 6 illustrates a sterilization device according to another preferredembodiment of the invention;

FIG. 7 illustrates the relative variation of the UV emission intensityas a function of the percentage of molecular oxygen present in a N₂/O₂mixture, at different flow rates, in a process for sterilizing acontaminated object according to another preferred embodiment of theinvention;

FIG. 8 illustrates the relative variation of the UV emission intensityas a function of the percentage of molecular oxygen present in a N₂/O₂mixture, at different pressures, in a process for sterilizing acontaminated object according to another preferred embodiment of theinvention;

FIG. 9A illustrates the relative variation of the UV emission intensityas a function of the percentage of molecular oxygen present in a N₂/O₂mixture, at different microwave power intensities, in a process forsterilizing a contaminated object according to another preferredembodiment of the invention;

FIG. 9B illustrates the relative variation of the UV emission intensityas a function of the percentage of molecular oxygen present in a N₂/O₂mixture, at same or normalized microwave power intensities, in a processfor sterilizing a contaminated object according to another preferredembodiment of the invention;

FIG. 10 illustrates the relative variation of the UV emission intensityas a function of the percentage of molecular oxygen present in a N₂/O₂mixture, in different sterilization chambers, during a process forsterilizing a contaminated object according to another preferredembodiment of the invention;

FIG. 11 illustrates the UV emission intensity relative variation alongthe discharge axis, for different N₂ flow rates with a fixed percentageof O₂ and at constant pressure, in a process for sterilizing acontaminated object according to another preferred embodiment of theinvention, wherein a N₂/O₂ mixture is used for sterilizing thecontaminated object;

FIG. 12 illustrates that in a process for sterilizing a contaminatedobject according to another preferred embodiment of the invention,maximum UV intensity at a given pressure is observed to depend on theflow rate of gas; and

FIG. 13 illustrates the number of spore B. subtilis survivors as afunction of time in a process for sterilizing a contaminated objectaccording to another preferred embodiment of the invention, wherein apost-discharge of argon is used to sterilize the contaminated object.

DESCRIPTION OF PREFERRED EMBODIMENTS

The following non-limiting preferred embodiments further illustrate theinvention.

FIG. 1 represents the intensity variation of UV photon emission,measured at 320 nm in the NO_(β) (250-380 nm) band, as a function of thepercentage of O₂ added to N₂ in the N₂/O₂ mixture that feeds thedischarge tube of a cylindrical sterilization chamber. This experimenthas been carried out by using according a device having 20 Lsterilization chamber as illustrated in FIG. 5. Such a device isprovided with a discharge tube whose inner diameter is 6 mm, and a highfrequency generator that emits a wave at the 2450 MHz frequency at anoutput power of 100 W. The chamber was operating at a pressure of 5torrs and a flow rate of 1 liter standard/minute (slm).

In FIG. 2, the intensity variation of UV photon emission, measured at320 nm in the NO_(β) (250-380 nm) band as a function of the percentageof O₂ added to N₂ in the N₂/O₂ mixture is shown. This experiment wascarried out by using a parallelepipedal sterilization chamber of 50Liters similar to the device illustrated in FIG. 6. The discharge tubehad an inner diameter of 26 mm. A high frequency generator that emits awave at the 915 MHz frequency at an output power of 300 W was used. Thechamber was operated at a pressure of 6 torrs, with a flow rate of 1slm.

As it can be seen in FIG. 3, the number of survivors will considerablyvary as a function of the O₂ percentage in the N₂/O₂ mixture. FIG. 3represents a diagram of spore B. subtilis survivors that have beenexposed to a post-discharge of N₂/O₂ at 0 and 0.2% of O₂. Sterilizationwas obtained in a chamber as illustrated in FIG. 6 in about 40 minutes,with 0.2% O₂. Sterility has been checked according to the method ofpositive tests (lozenge symbol). Time D indicated for each phaserepresents the time required to reduce spore population by 90%. Thechamber was operated at a pressure of 8 torrs, with a flow rate of 2slm, and at an output power of 300 W. It thus appears that at least aminimum of oxygen was required in order to have a reasonable time ofsterilization.

However, as shown in FIG. 4, Applicants have obtained surprising andvery interesting results by using a gas stream consisting of argon.These results are quite surprising in view of FIG. 3 were it has beenshown that the presence of O₂ seemed to be required. Moreover, in U.S.Pat. No. 6,707,254 the use of a discharge consisting of argon alone didnot lead to sterilization in a reasonable time, as shown in FIGS. 4 and6 of this prior art document. These results in U.S. Pat. No. 6,707,254have been obtained at a working pressure between 3.2 and 7 torrs at aflow rate of 2 slm. As it can be seen from FIG. 4 of the presentapplication, sterility was obtained quite rapidly (80 minutes) in achamber similar to the chamber illustrated in FIG. 6 by using an argongas stream, provided that the experiment was carried out with the rightcombination of gas pressure and flow rate, namely at 0.07 slm and 0.17torrs. At 3 torrs., sterility is not obtained at 2.8 slm since there arestill more than 500 spores to be inactivated after 80 minutes.

FIG. 5 shows the device as described in U.S. Pat. No. 6,707,254. Thisdevice 10 for sterilization comprises a plasma source 12, a N₂ mass flowcontroller 50, a N₂ source 52, an O₂ mass flow controller 54, an O₂source 56, a post-discharge chamber as the sterilization chamber 14, avacuum pump 16, and a pressure indicator 17 which controls the gaspressure. The plasma is produced by a microwave discharge at 2.45 GHz bypropagation of an electromagnetic surface wave at a maximal power of 180W from a generator 11. The power is measured from a power meter 13. Fora power of about 100 W or more, it is preferable, particularly if theobject to be sterilized is thermally sensitive, to cool the dischargetube 20 with compressed air. The surface wave is excited by aconventional surfatron 18. Any other device for exciting the surfacewave, such as those described in U.S. Pat. Nos. 4,043,940; 4,810,933, orin J. Phys. D.: Appl. Phys. 1991, 24, 1025-1048 may be used, whichallows the broadening of the range of possible frequencies from a fewMHz to some GHz. The diameter of the discharge tube 20 through which theplasma is injected in chamber 14 is not critical, but is preferablychosen in order to optimize the production of requested species for thesterilization. According to the embodiment illustrated in FIG. 5, theinternal diameter of the tube is of 8 mm and its length is 300 mm. Thedischarge tube may be fabricated with any material compatible with theoperation of the system, for example from fused silica.

In order to prevent overheating of vacuum sealing elements (o-rings)placed between chamber 14 and tube 20, which is generated by themicrowaves and the plasma, the diameter of tube 20 is expanded to 30 mmat extremity 22 which is adjacent to chamber 14. The gas(es) is (are)introduced into tube 20 at a rate which is adjusted with the help of aflowmeter (for example, 50 and/or 54) which is previously calibrated.The gases are conducted into chamber 14 by means of pump 16 generating aprimary vacuum (residual vacuum of 30 to 50 mtorr). A more or lessimportant throttling of the pump allows the gas pressure to be fixed inthe reactor. Evacuation of the gas from pump 16 is done outside thebuilding, eventually through appropriate filters. The pressure insidethe chamber is preferably reduced at a value situated between 30 and 50mtorr, and the pressure of the gas entering the chamber is adjustedpreferably between 1 and 7 torr.

Chamber 14 has a volume of 20 liters, and is made of Pyrex™ (i.e.,borosilicate glass), but can be made of any material compatible with thereaction mixture. Pyrex™, due to its transparency, is particularlyadvantageous since it allows observations by emission spectroscopy, tosee the effects of the positioning of the object to be sterilized on thegaseous flux, etc. Advantageously, the objects to be sterilized areplaced in a support 24 that is preferably made of stainless steel. Theform of the support is suited to facilitate its cleaning after use, aswell as the retrieval of the sterilized objects. In order to set thetemperature in the support 24, it is possible to circulate in it, in aclosed circuit, a cooling liquid inside a duct 26, also made ofstainless steel.

FIG. 6 illustrates a sterilization device 110 according to the presentinvention. The device has a source of plasma 112, a sterilizationchamber 114 of parallelepidal shape, and a discharge tube 120. Thechamber 114 is made of aluminum. Preferably, the chamber 114 is providedwith observation windows (not shown) so as to carry out opticalspectroscopy measurements. The device 110 also comprises a generator 118and supports 124 for receiving the objects to be treated. The chamberhas a capacity of 50 liters (known conventional sterilizers have acapacity of 50, 100 or 150 liters). The device 110 also comprisesseveral of the same elements as defined in FIG. 5 (such as pump, coolingduct, pressure gauge, surfatron, power meter etc. (not shown)) and isoperated in a similar manner. Uniformity of the active species in thesterilization chamber, which ensures that the objects are sterilizednotwithstanding their position in the chamber, and which minimizes highgas flow turbulence effects (it is then possible to operate at 2-3 slmwithout turbulence), has been obtained thanks to a tube whose wideneddischarge diameter is 26 mm (to achieve this, microwave frequency wasalso decreased from 2450 MHz to 915 MHz (R=0.09). Indeed, the device ofFIG. 5 (R=0.04) was truly efficient only in the vicinity of the axis ofthe discharge tube.

FIG. 7 shows the relative variation of the UV emission intensity as afunction of the percentage of O₂ in a N₂/O₂ mixture at different flowrates. The optimum percentage of oxygen, given the gas flow rate, isindicated on each curve. Intensity of the UV signal is collected at theinlet of the chamber of a device similar to the device of FIG. 6.

FIG. 8 shows the relative variation of the UV emission intensity as afunction of the percentage of O₂ in a N₂/O₂ mixture at differentpressures and for a fixed flow rate, in a sterilization chamber similarto the one illustrated in FIG. 6.

It can thus be seen from FIGS. 7 and 8 that by adjusting the flow rateand the pressure at optimum values, the intensity of Ultra Violet (UV)radiation can be maximized.

FIGS. 9A and 9B show the influence, in a sterilization chamber similarto the chamber of FIG. 6, of microwave power on UV emission intensityvariation as a function of the percentage of O₂ in a mixture N₂/O₂. InFIG. 9A, the UV emission is expressed in relative intensity and in FIG.9B the UV emission intensity is further expressed normalized to unity.

In FIG. 10 a comparison between two different chambers under similarconditions (1 slm of N₂, pressure 5 torrs) is represented. The relativevariation of the UV emission intensity is expressed as a function of thepercentage of O₂ in a N₂/O₂ mixture. In the 20 L cylindrical chambersimilar to the chamber of FIG. 5, power is 100 W, i.e., 5 W/L, while inthe 50 L parallelepipedal chamber power is 300 W, i.e., 6 W/L.Intensities are normalized to unity.

In FIG. 11, the results presented refer to the uniformity obtained inthe sterilization chamber along the discharge tube axis during thetreatment. The UV emission intensity is plotted at different nitrogenflow rates, with a fixed percentage of added oxygen (0.2%) in the N₂/O₂mixture and at a constant pressure of 8 torrs. At about 2 slm, theobserved UV intensity is relatively uniform along the chamber. Thechamber used is similar to the chamber of FIG. 6.

In FIG. 12, several tests have been made with gas streams consisting ofargon. In fact, various flow rates have been tested so as to check theinfluence on the UV emission intensity. It has thus been demonstratedthat the pressure at which maximum UV intensity is observed, depends alot on the flow rate of argon. If the flow rate is weak (□), theoperating pressure should be low. On the contrary, if the flow rate ishigh (▪), the optimum pressure should be high.

In fact, as shown in FIG. 12, an improvement of the sterilizationtreatment can be carried by obtaining by maximizing the Ultra Violet(UV) radiation intensity. By measuring the UV emission intensity for agiven flow rate (constant flow rate) as a function of pressure, apreferred range of values of pressure is thus obtained for such a givenflow rate. Such a measurement is also preferably done for at least 2 or3 other given values of flow rate (as example see the flow rates of 140,280, 1400, and 2800 mslm in FIG. 12) in separate experiments. Thus, suchexperiments permit to select parameters that allow to maximize the UVradiation intensity, thereby optimizing the sterilization treatment byeventually reducing the time required for obtaining sterility of thetreated object (see FIGS. 3, 4, and 13). The person skilled in the artwill also understand that alternatively, such an optimization of theprocess can be done by measuring the UV emission intensity for a givenpressure (constant pressure) as a function of the flow rate. Thus, apreferred range of values of flow rate is thus obtained for such a givenpressure. Such a measurement is also preferably done for at least 2 or 3other given values of pressure in separate experiments.

FIG. 13 shows a spore B. subtilis survival curve in a post-dischargeconsisting of argon, in a sterilization chamber chamber similar to thechamber of FIG. 6, In FIG. 13, given the pressure of 0.1 torrs, the gasflow was carefully adjusted to yield a UV emission intensity close tomaximum. FIG. 13, compared to FIG. 4, shows that optimizing UV intensityreduces sterilization time: sterility is attained in FIG. 13 in 40minutes while it required 80 minutes in FIG. 4 (0.07 slm, 0.17 torrs).

The following non-limiting examples further illustrate the invention.

EXAMPLES

Description of a Commercial Form of Plasma Sterilizing Device

The present device is presented by way of illustration only and shouldnot be interpreted as constituting any kind of limitation to the objectof the present invention. It comprises three main elements: a plasmasource, a sterilization chamber where the objects to be sterilized aredisposed (for example, on grids (acting as support)), and a vacuum pumpto: 1) initially evacuate the chamber to a residual pressure of about20-50 mtorr, and 2) thereafter maintain the desired pressure in thepresence of the gas flow rate while simultaneously making sure that theactive species are renewed by continuous evacuation of the gases. Thetested sterilization chambers were built of aluminum, which is a lightand inexpensive material. They may by cylindrical or parallelepipedal.Means for supporting objects (such as a grid) to be sterilized areintegrated in the chamber, and the latter is advantageously providedwith an access door allowing to load and unload the sterilizer.

The sources of plasma: 1) allow a sufficient UV radiation to achievesterilization in at most 40 minutes; 2) make sure that the activespecies in the chamber are uniform. To this end, the diameter of thedischarge tube with respect to the cross-section of the chamber(diameter that determines the upper working frequency) is kept as largeas possible, but actually not too large to make the device cumbersome orcostly. More than one source of plasma may be used, the latter beingadequately disposed, and the gas stream is adjusted to optimize thisuniformity; 3) make sure that the temperature does not exceed 50 to 60°C. in the chamber; and 4) utilize gases and operating conditions thatminimize damages caused to the objects by this sterilizing process (inparticular, utilizing as little O₂ as possible in the gas mixtures oravoid use of O₂).

The sterilizing system comprises a device ensuring the sterilizationcycle: evacuation of the chamber, gas delivery, pressure and flow ratecontrol, supplying microwave output and ignition of discharge; at theend of the cycle, return to atmospheric pressure. To control the goodoperation of the sterilizer, the physical parameters are checked (basevacuum, pressure and gas flow rate, incident and reflected microwavepowers, UV radiation intensity in the chamber in at least two points andrecording of this intensity all during the plasma cycle). Finally, atthe end of the cycle, the treatment parameters with traceability of thesterilized objects are printed.

The results which follow have been obtained in a parallelepipedalsterilization chamber (50 liters), except in FIGS. 1, 5, and 10 (inpart).

Determination of the Uniformity of Distribution of UV Radiation in theSterilization Chamber is Carried Out in the Examples in the FollowingManner.

The sterilization chamber FIG. 6 is provided in the present case with afused silica window, through which UV are transparent above 180 nm.Light emitted by a post-discharge volume element is collected, via acollimator, by an optic fiber that is connected to the input slit of theoptical spectrometer. The optical spectrometer (using a diffractiongrating) allows to record UV emission intensity of the post-discharge ata given wavelength. The collimator-optical fiber combination is designedso as to be movable in a plane according to the X and Y axes, thusmaking it possible to obtain a cartography, longitudinally andtransversely, of the UV emission intensity (normally recorded at 320nm).

Maximum UV Emission on the NO_(β) Band as a Function of the PercentageOf O₂ in the N₂/O₂ Mixture.

The results obtained, unless indicated otherwise, in a device similar tothe device shown in FIG. 6 including a single discharge tube andsupplied with a gas mixture consisting of N₂ and O₂. Ratio R=0.09.

Variation of the Optimum Percentage of O₂ with Respect to Flow Rate

The results reported in FIG. 7 show the relative variation of the UVemission intensity as a function of the percentage of added oxygen, withdifferent flow rates. On each curve, the optimum percentage of oxygen isspecified. Signal intensity is collected at the inlet of the chamber. Itis realized that the optimum percentage of molecular oxygen, forrealistic gas streams, is between 0.1 and 0.4 O₂ added to N₂ toconstitute the N₂/O₂ mixture.

Variation of the Optimal Percentage of O₂ with Respect to Pressure

The results reported in FIG. 8 show the relative variation of the UVemission intensity as a function of the percentage of oxygen added to N₂under different pressures. For a flow rate of 2 slm, the optimalpercentage is between 0.05% O₂ (10 torrs) and 0.3% (2.4 torrs) where thepressures under consideration are realistic.

Variation of the Optimal Percentage of O₂ with Microwave Power that isAbsorbed in the Discharge and Reported to the Capacity of theSterilization Chamber (1 to 8 W/liter)

The results reported in FIGS. 9(A and B) show the influence of microwavepower used on the relative variation of the UV emission intensity as afunction of the percentage of oxygen. (a) relative intensities; (b) sameintensities, but normalized to unity It is realized from FIG. 9(B) that,for a given pressure and gas stream, the power that is dissipated in thedischarge has only little influence on the relative variation of the UVemission intensity, although it affects its absolute value, as shown inFIG. 9(A).

Variation of the Optimal Percentage of O₂ with Respect to the Design andCapacity of the Chamber

The two chambers, studied by way of comparison in the present example,are those represented in FIGS. 5 and 6. The results reported in FIG. 10compares the relative variation of the UV emission intensity as afunction of the percentage of molecular oxygen in the two chambers: 1slm N₂—5 torrs; Pyrex chamber (5 W/L) vs parallelepipedal (6 W/L).Intensities are normalized. A study of the UV emission intensityvariation profile as a function of the percentage of O₂ added to N₂shows a maximum for the same percentage in the two chambers(independence of operating parameters, namely microwave frequency,diameter of the discharge tube, geometry and nature of the chamber,ratio R, position of the pumping orifice,); on the other hand, adecrease of UV intensity is carried out less rapidly in the cylindricalchamber. It should be noted that the cylindrical chamber FIG. 5 is fedwith a discharge at 2450 MHz while the parallelepipedal chamber is fedwith a 915 MHz discharge. Conclusion: in the examples shown, the optimalpercentage is between 0.05 and 0.4% O₂ added to N₂ thus covering a widerange of variation of the operating parameters (microwave frequency,flow rate, discharge tube diameter,).

Improvement of Spatial Distribution Uniformity of UV Photon EmittingSpecies in the Sterilization Chamber

The results which follow have been obtained in a device similar to the50 liter parallelepipedal chamber schematically illustrated in FIG. 6.Measurements were carried out by optical emission spectroscopy at awavelength (320 nm) which is representative of the NO_(β) band emissionintensity.

The results that are reported in FIG. 11 represent UV emission intensityvariation along the axis of the discharge tube into the rectangularchamber, for different flow rates of nitrogen, with fixed percentage ofadded oxygen (0.2%) and under constant pressure. At about 2 slm, UVintensity is relatively uniform along the chamber.

For a given pressure (or in the immediate neighborhood) under optimum O₂for UV emission intensity, a control of the flow rate of N₂ (here at 2slm) makes it possible to obtain a uniform UV emission along the chamber(plasma discharge is located on the left of FIG. 11). If uniformity isintended at another flow rate, the pressure inside the chamberpreferably has to be modified accordingly.

Inactivation of the Spores with a Post-Discharge of a Gas Stream of PureArgon (or Consisting of Argon)

The results reported in FIG. 12 show absorption of the argon line at763.5 nm as a function of pressure for different flow rates.Measurements were made at 5 cm from the inlet into the chamber by meansof an optical spectrometer. The intensity of this UV line for argondepends on the density of the resonant states that emit these photons.The density of these states is obtained by optical absorptionmeasurements: the higher the absorption, the higher the UV intensityemitted. The most direct way to optimize UV emission in the case of asingle gas such as pure argon is to cause the pressure to vary. FIG. 13shows in fact that the pressure at which maximum UV intensities areobserved depends on the flow rate of argon. If the latter is low (e.g.70 mslm), the operating pressure is preferably low (here about 0.1torr). On the contrary, if it is high, the optimal pressure ispreferably higher. The survival curve of the B. subtilis spores isrepresented in FIG. 13 and confirms the results of FIG. 4. By adjustingthe pressure and the flow rate of argon, it is possible to have a higheryield of UV emission intensity, which makes it possible to achievesterilization in a shorter period of time. The results reported in FIG.13 show the survival curve of B. subtilis spores in a post-discharge ofpure argon, as obtained in the conditions indicated, and in the presenceof an intensity of UV emission close to a maximum value.

A third sterilizer (not illustrated), in accordance with the presentinvention, of parallelepipedal structure and made of aluminum, andhaving a capacity of 60 liters, is supplied with a plasma produced at200 MHz in a tube whose inner diameter is 48 mm. The results obtainedare comparable to those collected with a 50 liter parallelepipedalsterilizer operating at 915 MHz. To increase the uniformity ofdistribution of the active species in the sterilization chamber, thelatter is supplied from two plasma sources, which are ideally separatedfrom one another. On the other hand, the orifice of evacuation for thegases is placed, in the present case, in the axis of the chamber, whilein FIG. 6 pumping is carried out at the bottom of the chamber.

It has thus been shown that by controlling the flow rate and/or the gaspressure in the chamber, it is possible to maximize the Ultra Violet(UV) radiation intensity once a given range of pressure and/or gas flowhas been chosen. Moreover, It has been found that for the samesterilization chamber, and for the same frequency, the surface of theobjects to be sterilized may be increased considerably through aselection of certain structural and energetic parameters of the system.It also has been shown that the axial homogeneity of the plasma isinfluenced by the flow rate and pressure, as well as the feed gas thatis used to produce the plasma. It also has been found that thetransverse and axial homogeneity of the plasma flow is dependent on astructural ratio R, which is indicative of the difference of size of thedischarge tube with respect to that of the sterilization chamber, R ishereinafter defined. To increase transverse homogeneity, for example,the ratio R must be increased. The upper limit of ratio R can bedetermined by the diameter of the discharge tube that is used. Themaximum diameter of the discharge tube can be increased when thefrequency of the electric field is decreased. On the other hand, thereis a lower limit of frequency of the electric field with respect to theefficiency of HF power transfer towards the discharge. Consequently, itis advantageous to harmonize tube diameter and plasma source frequencyso as to obtain the highest homogeneity possible of the sterilizingspecies. Moreover, it has been shown that by using a gas streamconsisting of argon it is possible to obtain sterility of thecontaminated object and that such a treatment avoids to expose thecontaminated object to oxidizing species so that deterioration orerosion of the object is considerably reduced.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodifications and this application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departures from thepresent disclosure as come within known or customary practice within theart to which the invention pertains and as may be applied to theessential features hereinbefore set forth, and as follows in the scopeof the appended claims.

1. Process for sterilizing a contaminated object comprising: insertingthe object in a sterilization chamber having at least one discharge tubein communication therewith, feeding the tube with a liquid or gasstream, and submitting the stream to an electric field so as to generatea plasma, thereby exposing said contaminated object to the action ofsterilizing species that are present in a post-discharge zone or in azone of excitation of said plasma, wherein said stream is adjusted bycontrolling its flow rate and/or gas pressure in the chamber, so as tomaximize Ultra Violet (UV) radiation intensity.
 2. Process according toclaim 1, wherein the gas stream consists of helium, neon, argon,krypton, xenon, or mixtures thereof.
 3. Process according to claim 1,wherein the gas stream consists of argon.
 4. Process according to claim1, wherein the object is contaminated with micro-organisms selected fromthe group consisting of viruses, spores, bacteria, fungi, molds, andprions.
 5. Process according to claim 1, wherein the electric fieldfrequency is from 10 Megahertz to 3 Gigahertz.
 6. Process according toclaim 5, wherein said frequency is from 100 to 2450 MHz.
 7. Processaccording to claim 1, wherein the gas stream flow rate is between 10 and5000 standard cm³ per minute.
 8. Process according to claim 7, whereinthe gas stream flow rate is between 50 and 3000 standard cm³ per minute.9. Process according to claim 1, wherein the pressure generated insidethe sterilization chamber is between 0.05 and 10 torrs.
 10. Processaccording to claim 1, wherein the gas stream comprises argon and thepressure generated inside the sterilization chamber is between 0.1 and 4torrs.
 11. Process according to claim 1, wherein the gas streamcomprises at least one component selected from the group consisting ofmolecular oxygen, nitrogen, neon, argon, krypton, xenon, helium, oxygen,carbon monoxide, carbon dioxide, N₂O, gases of formula NO_(x) wherein xrepresents 1, 2 or 3, air, and mixtures thereof.
 12. Process accordingto claim 11, wherein the gas stream comprises molecular oxygen. 13.Process according to claim 12, wherein the gas stream comprises at least0.04% of molecular oxygen.
 14. Process according to claim 12, whereinthe gas stream comprises from 0.1 to 10% of molecular oxygen. 15.Process according to claim 1, wherein the gas stream comprises molecularoxygen and a component selected from the group consisting of nitrogen,neon, argon, krypton, xenon, helium, oxygen, carbon monoxide, carbondioxide, N₂O, gases of formula NO_(x) wherein x represents 1, 2 or 3,air, and mixtures thereof.
 16. Process of sterilization according toclaim 11, wherein the gas stream comprises molecular oxygen, nitrogen,argon, and helium.
 17. Process of sterilization according to claim 11,wherein the gas stream comprises molecular oxygen, nitrogen, argon, andnitrogen dioxide.
 18. Process according to claim 1, wherein the stepsinvolve a pulsed gas in an electric field that is applied continuously,a pulsed electric field in a continuous gas stream, a pulsed gas in asynchronously pulsed electric field, a gas change, or a combination ofthese steps.
 19. Process according to claim 1, wherein the ratioR=(CDT)/(CSC) is comprised between 0.01 and 0.70, where CDT representsthe cross-section of the discharge tube or the sum of the cross-sectionsof the discharge tubes, and CSC represents the cross-section of thesterilization chamber.
 20. Process for sterilizing a contaminated objectin a sterilization chamber provided with at least one discharge tube,wherein the discharge tube(s) is in communication with the sterilizationchamber and is (are) supplied with a liquid or gas stream, thecontaminated object is subjected in the sterilization chamber to theaction of sterilizing species that are present in a post-discharge zoneor in the excitation zone of the plasma that is generated at the levelof the discharge tube(s) by passing the stream in an electric field,wherein the ratio R=(CDT)/(CSC), in which CDT represents thecross-section of the discharge tube in communication with thesterilization chamber or the sum of the cross-sections of the dischargetube(s), and CSC represents the cross-section of the sterilizationchamber, confirms the relation 0.05<R<0.70, and wherein the stream isadjusted by controlling the flow rate and/or gas pressure in thechamber, so as to maximize Ultra Violet (UV) radiation intensity. 21.Process according to claim 20, wherein 0.09≦R≦0.60.
 22. Processaccording to claim 20, wherein 0.2≦R≦0.40.
 23. Process according toclaim 20, wherein the gas stream flow rate has a value that is comprisedbetween 10 and 5000 standard cm³ per minute.
 24. Process according toclaim 23, wherein the gas stream flow rate is comprised between 50 and3000 standard cm³ per minute.
 25. Process according to claim 20, whereinthe pressure generated inside the sterilization chamber is between 0.05and 10 torrs.
 26. Process according to claim 20, wherein the gas streamcomprises argon and the pressure generated inside the sterilizationchamber is between 0.1 and 4 torrs.
 27. Process according to claim 20,wherein the gas stream consists of helium, neon, argon, krypton, xenon,or mixtures thereof.
 28. Process according to claim 20, wherein the gasstream consists of argon.
 29. Process according to claim 20, wherein thegas stream comprises at least one component selected from the groupconsisting of molecular oxygen, nitrogen, neon, argon, krypton, xenon,helium, oxygen, carbon monoxide, carbon dioxide, N₂O, gases of formulaNO_(x) wherein x represents 1, 2 or 3, air, and mixtures thereof. 30.Process according to claim 20, wherein the gas stream comprisesmolecular oxygen and a component selected from the group consisting ofnitrogen, neon, argon, krypton, xenon, helium, oxygen, carbon monoxide,carbon dioxide, N₂O, gases of formula NO_(x) wherein x represents 1, 2or 3, air, and mixtures thereof.
 31. Process according to claim 30,wherein the gas stream comprises from 0.1 to 10% of molecular oxygen.32. Process according to claim 20, wherein the gas stream comprises N₂and molecular oxygen, the percentage of molecular oxygen in the streambeing adjusted to a content x, of molecular oxygen, such that 0<x<0.5%.33. Process according to claim 32, wherein x is varying from 0.1 to0.4%.
 34. Process according to claim 32, wherein the molecular oxygen isat least partially converted into atomic oxygen.
 35. Process accordingto claim 20, wherein the sterilization chamber is perpendicular to thedirection of the gas stream feeding the discharge tube and cross-section(CSC) representing the cross-section of the chamber in communicationwith the discharge tube and which is perpendicular to the plasmacurrent.
 36. A device allowing the implementation of the process definedin claim 20, comprising a source of plasma associated with one of thewalls of the sterilization chamber by means of at least one dischargetube in which there is injected a gas or a mixture of gases eventuallyproducing the plasma, the chamber comprising the object to besterilized, and a vacuum pump to carry the gases in the chamber and tomaintain therein a reduced pressure, wherein the source of plasmacomprises an electric field applicator and the ratio R=(CDT)/(CSC), inwhich (CDT) represents the cross-section of the discharge tube or thesum of the cross-sections of the discharge tube(s) in contact with thesterilization chamber and (CSC) represents the cross-section of thesterilization chamber (CSC), confirming the relation 0.05<R<0.70